1. Field of the Invention
The present invention relates to the fabrication of semiconductor integrated circuits (IC's). More particularly, the present invention relates to methods and apparatuses for determining wafer trench depth.
2. Description of the Related Art
One of the operations in the fabrication of IC's is the etching of trenches into the surface of silicon wafers. This etch operation is typically performed using well known photolithography and plasma etch technology. Generally, the desired depth of these trenches ranges from about 0.1 microns to 3.0 microns with control of the precise depth of the trench being an important consideration.
During the plasma etch process, the etch rate can vary as a function of etching variables such as the chamber component temperature, the chamber conditioning, and the wafer resist age. Improved trench depth control can result if this “process drift” can be monitored and compensated.
Currently, there are several well know optical techniques used to measure the depths of trenches etched into silicon, such as spectral reflectometery, monochromatic interference, laser triangulation, confocal imaging, and phase contrast.
While these methods are potentially usable, each of them is limited in some way by the physics and specific limitations of the techniques. For example, the interpretation of spectral reflectometery data requires either a prior knowledge of: film thickness', materials, and refractive indexes, or, complex and error prone “fitting” techniques. The inaccuracies in the optical modeling techniques used are reflected as errors in the calculated etch depth. A change or drift in the refractive index of a film could be interpreted as an error in the trench depth and an erroneous control action taken. A trench depth monitoring capability that is excessively sensitive to extraneous variables may actually result in increased variability, and therefore error rate.
In view of the foregoing, what are needed are improved methods and apparatuses for detecting etch trench depth. Further, the methods should be sensitive only to distance, measure an average depth over a reasonable area, and be compact, robust, and cost effective.